The porous bottom culture dishes (PBCD's) and apparatus for growth, differentiation and sterile measurement of epithelia in culture which have been developed in this laboratory are being used in many laboratories world wide. The porous membranes of the PBCD's allow nutrient and waste exchange and passage of ions at the basolateral or blood equivalent side of the epithelial cell layer. This allows more normal cellular function than occurs on petri dishes and the easy sterile measurement of active sodium transport. This transport is a basic differentiated function which has previously been difficult to measure quantitatively. In fact, most cell culture investigations have focused on cell growth rather than differentiated function. As a result, most cell culture media are developed to give maximal growth rather than optimal differentiated function. Because of this and to avoid variability caused by lot to lot variation in the fetal bovine serum (FBS) used in most media, work on a defined medium for optimal transport was undertaken. Such a medium for the A6 cell line (from xenopus laevis kidney) was developed. In order to obtain stable transport instead of maximal growth, the insulin which is used in most defined medium was replaced with epidermal growth factor. In addition, it was found essential to coat the porous membranes (cellulose esters) with fibronectin. Additional techniques have been developed for making transparent collagen membranes for use on the PBCD's. High purity rat-tail collagen yields improve optical properties but no apparent change in A6 cell behavior. This work stimulated the development of collagen catheters to be used as topocather extensions on conventional catheters.